In the manufacture of multi-layer glass containers, it is desirable to measure the thickness of the glass layers in order to control the quality of the containers. In so-called cased glass containers, an internal relatively thick layer of core glass is surrounded by an external relatively thin layer of casing glass. The thermal characteristics of the core and casing glasses are such that the outer casing glass layer is placed under compressive stress as the glass container cools. This compressed glass outer layer enhances the strength of the overall container, and allows manufacture of containers having desired strength properties using less total glass.
In order to control the manufacturing process from both a cost and quality standpoint, it is necessary to measure and control the thickness of the casing glass layer. In the past, the thickness of the casing glass layer has been measured by cutting a section from the container sidewall and measuring the glass layer thicknesses using a microscope or the like. This technique is expensive to implement, and is not well suited for use as a real-time quality control technique in mass production of containers. It has been proposed to employ electro-ol techniques to measure thickness of the internally stressed casing glass layer using polarized light. Transmission of the polarized light tangentially through the casing glass layer of the container sidewall causes a birefringence pattern, which can be analyzed to determine casing glass layer thickness. A container under test is vertically immersed in a bath of oil, and the finish of the container is coupled to appropriate means for rotating the container in the bath about its central axis. A polarized light source is disposed to direct light energy through the bath along an axis tangential to the sidewall of the container onto a sensor. The oil is needed to match the index of refraction of the container sidewall so that the light energy travels tangentially through the container sidewall to the sensor, rather than being reflected from the sidewall onto the wall of the tub containing the bath. The sensor includes appropriate means responsive to birefringence in the light energy incident thereon for measuring the thickness of the internally stressed outer glass layer of the container.
The technique so described does not provide satisfactory results. Glass containers are not always symmetrical about their axes of rotation, and the body of the is container is not always coaxial with the container neck or finish. Thus, if the container is out-of-round or has a body axis that is non-coincident with the axis of the finish, the sidewall of the container will wobble with respect to the light path as the container is rotated in the index oil. This wobble may be on the order of one-half inch, as compared with a desired measurement accuracy on the order of 0.001 inch. Furthermore, the described technique leads to substantial wastage of expensive index oil. It is therefore a general object of the present invention to provide a method and apparatus for optically measuring the thickness of a casing glass layer in a cased glass bottle of the described character by measuring stress in the container sidewall, in which the container is accurately positioned and held in position during the measurement process. Another object of the present invention is to provide a method and apparatus of the described character in which the amount of index oil used is greatly reduced, and which are adapted to measure casing glass thickness at varying position both axially and circumferentially around the container. Another object of the present invention is to provide a method and apparatus that satisfy the foregoing objectives, that are adapted to obtain rapid and accurate measurement of casing glass thickness, and that can thus be used for real-time control of the glassware manufacturing process.